1. Field of the Invention
The present invention relates, in general, to a mesoporous molecular sieve substance and methods for preparing the same. More particularly, this invention relates to a mesoporous molecular sieve substance having a structure exhibiting a hexagonal arrangement of straight channels uniform in diameter, superior over MCM-41 manufactured by Mobil Co. in thermal stability and hydrothermal stability, and is concerned with methods for preparing the mesoporous molecular sieve substance.
2. Description of the Prior Art
Generally, porous substances are divided by pore size, for example, pore sizes smaller than 2 nm classified as microporous substances, between 2 and 50 nm classified as mesoporous substances and larger than 50 nm classified as macroporous substances. Of the porous substances, those having uniform channel, such as zeolite, are defined as molecular sieves and up to hundreds of types of species have been found and synthesized thus far. Zeolites play an important role as catalysts or carriers in modern chemical industries by virtue of their characteristics including selective adsorptivity, acidity and ion exchangeability. However, the molecular size of a reactant which can be utilized in catalyst conversion reactions, etc. is limited by the pore size of zeolite because zeolite is a microporous molecular sieve. For example, when ZSM-5 zeolite is applied in a catalytic cracking reaction, its reactivity becomes significantly decreased as the reactant changes from n-alkane to cycloalkane and further to branched alkane. Hence, an enormous effort has been made all over the world to synthesize molecular sieves having larger pores than that of zeolite. As a result, AlPO.sub.4, VPI-5, Cloverlite and JDF-20 having larger pore size than that of zeolite were developed by many researchers. However, these molecular sieves cannot exceed the microporous limit.
Recently, a group of researchers in Mobil Co. reported a series of mesoporous molecular sieves, named MCM-41, in U.S. Pat. Nos. 5,057,296 and 5,102,643. According to these patents, MCM-41 has a structure exhibiting hexagonal arrangement of straight channels, such as honeycomb, on a silica plate. Recent research reports illustrate that MCM-41 is synthesized through a liquid crystal template pathway. That is, in an aqueous solution, surfactants form a liquid crystal structure which is surrounded by silicate ions and the liquid crystal structure is associated with MCM-41 substance via a hydrothermal reaction and then, removed by calcination at a temperature of 500 to 600.degree. C., to prepare MCM-41. At this time, its pore size can be adjusted in a range of from 1.6 up to 10 nm by modulating the kinds of surfactants or synthesis conditions. Thus, MCM-41 is of mesoporosity with larger pore size than that of existing molecular sieves, such as zeolite, and is being actively researched for characterization and application in many laboratories. Since MCM-41 is a mesoporous molecular sieve, which has regularly arranged channels, it is used for study on adsorption characteristics of various gases as well as used as a carrier for conductive polymer, organometallic compound and poly acid by virtue of its large pores. Further, another type MCM-41 whose framework atoms are partly substituted with other elements, such as aluminum, boron, manganese, iron, vanadium, titanium, etc, has been developed for application to apply for a catalytic reactions of macro organic compound or for use in catalytic carriers.
As mentioned above, the application examples of MCM-41 in various fields are reported. However, one of the most important physical properties of MCM-41 for the application is thermal stability. According to the early researchers, after being subjected to calcination to remove template materials, MCM-41 samples undergo structure contraction ranging from 20 to 25% relative to pre-calcination, although exhibiting a little difference depending on synthesis conditions. This contraction is attributed to the fact that the silanol groups are condensed by the calcination. However, recent research data shows that, if the equilibrium of silicate condensation reaction is moved toward the product by controlling pH of the reactants during the hydrothermal synthesis of MCM-41, the condensation of the silanol group is already completed, so that the weak thermal stability can be surmounted and a considerable improvement can be brought into the uniformity of structure. The MCM-41 thus obtained was reportedly found not to undergo structural decomposition even at 500.degree. C. under 1 atm in the presence of 100% water vapor, in addition to being only slightly contracted in structure even upon heating up to 900.degree. C. under an oxygen atmosphere.
Superior in thermal stability as it is, the MCM-41, however, starts to be slowly destructurized in water heated at 60.degree. C. or higher, which is ascribed to the hydrolysis of the silicate constituents. 12 hrs after being heated in boiling water of 100.degree. C., the MCM-41 completely loses its structural characteristics. This poor hydrothermal stability serves as a serious limiting factor in the case of requiring 60.degree. C. or higher, for example, in using a titanium-substituted molecular sieve in a partial oxidation reaction, in a catalyst conversion reaction needing a hydrothermal condition in which hydrogen peroxide is used as an oxidizing agent or in impregnating in the molecular sieve a transition metal, such as platinum, as a catalytically active ingredient.
Therefore, active research and development efforts have been made for an improvement in the hydrothermal stability of mesoporous molecular sieves.